SOPHEREUS. I have more than once referred to the fact that Nature does not permit crosses between the true species of animals, in breeding, and that we have no reason to suppose it ever did. This is a very important fact to be considered in weighing the claims of your theory of evolution. I have been looking into Darwin, and I find it somewhat uncertain in what sense he uses the terms "species," "races," and "varieties." In his "Descent of Man," he devotes a good deal of space to the discussion of the various classifications made by different naturalists under these respective terms; and there is no small danger of confusion arising from the use of these terms unless they are defined. The possibility of the process of evolution, as a means of accounting for the existence of any known animal, depends in some degree upon the animals among which, by sexual generation, the supposed transition from one kind of animal to another kind has taken place. Darwin speaks of the difficulty of defining "species"; and yet it is obvious (is it not?) that the theory of the graduation of different forms into one another depends for its possibility upon the forms which have admitted of interbreeding. While, therefore, the term "species" is in one sense arbitrary, as used by different naturalists, and there is no definition of it common to them all, it is still necessary to have a clear idea of the limits within which crosses can take place in breeding, because there are such limits in nature. Thus, in the case of man, as known to us in history and by observation, there are different families, which are classed as "races." Darwin speaks of the weighty arguments which naturalists have, or may have, for "raising the _races_ of man to the dignity of _species_." Whether this would be anything more than a matter of scientific nomenclature, is perhaps unnecessary to consider. Whether we call the "races" of men "species," or speak of them as families of one race, we know as a fact that interbreeding can take place among them all, and that between man and any other animal it can not take place. The same thing is true of the equine and the bovine races and their several varieties. Whether, in speaking of the different families or races of men, we consider them all as one "species," or as different species--and so of the varieties of the equine or the bovine races--the important fact is, that there are limits within which interbreeding can take place, and out of which it can not take place. Do you admit or deny that the barriers against sexual generation between animals of essentially different types, which are established in nature, are important facts in judging of the hypothesis of animal evolution?